In order to increase the transmission capacity of optical fibre networks and communication links, wavelength division multiplexing techniques are often utilised. In WDM systems, a plurality of wavelength channels are transmitted through a single optical fibre (or possibly, other waveguiding means). Due to the nature of light, cross-talk between different wavelength channels is negligible and very high transmission rates can be achieved.
Although current optical fibres exhibit extremely low losses, optical repeaters are needed if transmission is carried out over long-haul transmission lines. Such optical repeaters typically comprise fibre amplifiers for amplifying the optical signal propagated by the fibre. The amplification in the amplifiers is normally slightly different for different Wavelengths, i.e. for different channels within the WDM signal. Moreover, different wavelengths may experience different losses along the optical network. Therefore, the relation between the power levels of the individual channels is not preserved throughout the network.
It is often desirable to maintain a constant power distribution between the channels, or to maintain a constant signal-to-noise ratio between the channels. In other cases, it can be desirable to maintain a certain power profile between the different wavelength channels in a WDM signal. To achieve this, it is necessary to introduce “gain equalisation”, also known as channel balancing. A small variation between the channel powers at the transmitter can cause large variation between the channel powers at the other end. In general, the power levels of the different channels will change unpredictably as the signal propagates through the optical network. To overcome this problem, different approaches have been proposed.
A first suggestion was to utilise pre-ecualisation of the channels. Pre-equalisation is a method where the WDM signal is conditioned prior to transmission through the network. Compensation is then introduced at the transmitter in order to allots for upcoming channel-specific losses in the network. However, this method has some serious drawbacks. The amount of pre-equalisation possible is limited and may not be enough for the intended purposes. Furthermore, there has to be communication back to the transmitter from the receiver. In particular, this approach is only feasible in point-to-point connections where the routing through the network is the same for every channel in the WDM signal.
To improve the channel balancing, it was proposed to equalise the channels at every amplifier stage. This could be done by demultiplexing the channels after each amplifier stage, then attenuating each channel separately, and subsequently multiplexing the channels back together for further transmission through the network. However, this approach required a very complex structure of hardware, and added tolerance penalties in the demultiplexing/multiplexing stages. For these reasons, this approach was not feasible for commercial use.
Alternatively, a multi-channel filter could be used. One example of such filter is an acousto-optic tuneable filter (AOTF). An AOTF uses an RF signal to attenuate each channel independently of the others. However, the AOTF requires high amounts of driving power to balance more than a few (2–4) channels, and there is an increased tendency to cross-talk between the channels due to the acousto-optic filtering. Consequently, the use of an AOTF is not feasible when trying to equalise a large number or channels in a DWDM signal.
Another method is to use carefully designed passband filters to equalise the channels. Since this method is not tuneable, it is of minor importance in high-performance optical networks. Furthermore, fixed filters are believed to cause difficulties when used in connection with dynamic routing of channels, in which case it is not known in advance which way a certain channel will travel through the network.
A recent effort towards channel balancing is disclosed in U.S. Pat. No. 5,134,034. In that case, feedback from a receiver to a transmitter controls the power levels of individual channels before multiplexing the channels together. In essence, this is just pre-equalisation. As mentioned above, serious problems are connected with such an approach. Firstly, the method is only feasible for point-to-point, connections. If the network is constituted by a mesh, in which the channels can take any route, the principle does not apply. Secondly, and as mentioned above, pre-equalisation is sometimes not enough. This is particularly obvious when it is not known at the transmitter which route each channel will take through the network.
Consequently, there is a need for new methods and apparatus for channel balancing that can allow any routing through the network, that can be located at any point in the network, and that dynamically equalise the power levels between channels according to any desired scheme.